An MRI device is designed so that all or part of a patient's body is inserted into the cavity of a magnetic field generator that forms a powerful magnetic field, and a body section image of the desired area is obtained, allowing a graphic representation to be made even of the texture of the tissue of this area.
With the above-mentioned MRI-use magnetic field generator, the cavity must be wide enough for all or part of the patient's body to be inserted, and a stable, powerful, and uniform magnetic field having a precision of at least 1.times.10.sup.-4 at 0.02 to 2.0 T usually must be formed within the imaging space inside the cavity for a sharp body section image to be obtained.
FIGS. 9A and 9B illustrate a known structure of a magnetic field generator used in an MRI device (Japanese Patent Publication H2-23010). Specifically, permanent magnets 30, which make use of R--Fe--B-based magnets as the magnetic field generation source, are fixed to the opposing sides of a pair of base yokes 35, pole pieces 31 are fixed opposite one another on the various magnetic pole sides, and a static magnetic field is generated within the cavity 33 between the pole pieces 31. The illustrated magnetic circuit is achieved by connecting columnar support yokes 36 between the pair of flat base yokes 35. 37 in the figure is a tilt field coil, and 38 is an imaging space formed in the center within the cavity 33.
The pole pieces 31 are usually made from a flat bulk material (a single piece) produced by planing down electromagnetic soft iron, pure iron, or another such magnetic material. A structure in which an annular protrusion 32 is provided around the periphery, or a protrusion is provided in the center (not shown) (Japanese Utility Model Publication H5-37446), or the like may be employed in order to enhance the uniformity of magnetic field distribution within the cavity 33.
Because of their relatively low maintenance costs, compact size, and other advantages, permanent magnets are increasingly being utilized as the magnetic field generation source for forming a static magnetic field within the cavity 33. A drawback to these permanent magnets, however, is that the field intensity tends to vary with changes in the temperature due to the magnetic characteristics inherent to the magnets themselves.
The stability of the intensity of the static magnetic field formed in the cavity is important with an MRI device, and ways of keeping the field intensity stable include covering the entire magnetic field generator, or just the required portions, with an adiabatic material so that the permanent magnets are kept at a constant temperature, and providing cooling or heating means on the inside of the base yokes or the above-mentioned adiabatic material.
For instance, with a known structure in which a cooling means is provided in order to reduce the effect that temperature changes have on the static magnetic field generated by an MRI device, the temperature is controlled with a cooling apparatus in which electronic cooling elements that utilize the Peltier effect are disposed around the outer periphery of the base yokes (Japanese Utility Model Publication H3-56005). Specifically, the above-mentioned cooling apparatus cools the entire magnetic field generator to within a temperature range that is 10 to 50.degree. C. lower than the ambient temperature, changes in the ambient temperature are moderated by an adiabatic material that surrounds the device, and the temperature is fine-tuned to a specific range.
During diagnosis with an MRI device, the room temperature is usually kept at about 22 to 25.degree. C. so that a clothed patient can be examined in comfort. The structure described above requires that the MRI device be kept at all times at a temperature lower than room temperature, but this is impractical because of inefficiency from the standpoint of energy consumption, and because the structure for cooling the entire structure makes the device larger and more expensive than with the structure discussed below in which a heating means is provided.
A structure in which a heating means is provided makes it easier to obtain a compact and inexpensive device than the above-mentioned structure in which a cooling apparatus is provided, and is said to be more efficient in terms of energy consumption. Examples of such structures are known from Japanese Laid-Open Patent Applications S63-43649 and S63-278310.
Specifically, it is common to use a structure in which any of various heating means are used to control the entire magnetic field generator to a temperature that is about 5 to 10.degree. C. lower than the room temperature where the MRI device is installed.
The magnetic field generator shown in FIG. 10 is structured such that flat base yokes 42 are disposed across from one another via columnar support yokes 43, permanent magnets 40 are fastened to the opposing sides thereof, and pole pieces 41 are provided to the magnetic pole sides thereof. A planar heater 44 is disposed on the outer surface of each of the base yokes 42, a planar heater (not shown) is also disposed on the inner surfaces of the adiabatic materials 45, and these yokes are entirely covered with the adiabatic material 45.
With a structure such as this, electrical current is sent from a power source (not shown), and the temperature of the magnetic circuit is controlled.
Japanese Laid-Open Patent Application S63-43649 proposes a structure in which planar heaters are disposed only on the inner surfaces of the above-mentioned adiabatic materials 45. The problem with this structure, though, is that the temperature of the magnetic circuit is controlled by using a fan to forcibly send air heated by the planar heater through an air passage formed between the flat base yoke 42 and the adiabatic material 45, so not only is the device complicated, but because the magnetic circuit is heated via air, the thermal efficiency is also poor.
An object of the invention of Japanese Laid-Open Patent Application S63-278310 is to solve the above problems, and as shown in FIG. 10, the thermal efficiency is improved somewhat by directly disposing the planar heaters 44 on the outer surfaces of the base yokes 42 on which the permanent magnets 41 are disposed. However, because the heaters 44 are disposed on the outer surfaces of the base yokes 42, that is, on the sides opposite from the cavity-facing sides of the permanent magnets 40, there is a pronounced tendency for the heat to be diffused from the magnetic circuit to the outside, so no improvement in thermal efficiency is realized.
Furthermore, Japanese Laid-Open Patent Application H8-266506 (U.S. Pat. No. 5,652,517) discloses a structure that improves on the invention discussed in Japanese Laid-Open Patent Application S63-278310. The structure of Japanese Laid-Open Patent Application H8-266506 is characterized in that a thermally conductive material is attached, either directly or via a gas, to the side faces of upper and lower base yokes to which permanent magnets are attached.
The heater means in Japanese Laid-Open Patent Application H8-266506 is in the form of a sheet heater, and an AC sheet heater and a DC sheet heater are fixed one above the other to the side faces of the base yokes. The fixing is accomplished by covering the AC sheet heater and the DC sheet heater with a fixing bake plate from above and bolting the plate down.
Japanese Laid-Open Patent Application H8-266506 states that the above structure affords improvements in thermal efficiency, control follow-up properties, and ease of work as compared to the structures disclosed in Japanese Laid-Open Patent Applications S63-43649 and S63-278310.
Still, because even the structure in Japanese Laid-Open Patent Application H8-266506 makes use of planar heaters, there is fundamentally a great deal of thermal radiation to the sides opposite the sides in contact with the yokes, so thermal efficiency is poor. Also, it is indicated that a temperature sensor is only disposed in the vicinity of the center on the top face of the upper base yoke, and that the temperature of all the planar heaters is controlled according to the temperature detected by this lone temperature sensor. In other words, this structure involves controlling the temperature of the entire magnetic circuit with a single control system, so there is a wide temperature variance, and uniformity of the magnetic field is also lost.